the small brown nut of the oak tree, that grows in a base that is like a cup 橡子；橡实 
Word OriginOld English æcern, of Germanic origin; related to Dutch aker, also to acre, later associated with oak and the plant corn.
Idioms
great/tall oaks from little acorns grow
- (saying)
something large and successful often begins in a very small way 参天橡树长自小小橡实；合抱之树，生于毫末